Question

First of all, a snippet of my code in matrix.hpp:

template <class T>
class matrix
{
  public:
    matrix();

    T& operator() (size_t i, size_t j) {return elem[i+j*4];}
    const T& operator() (size_t i, size_t j) const {return elem[i+j*4];}

    // ...

    static matrix<T> identity() {return matrix<T>();}
    static matrix<T> translation(const vector<T>&);
    template <class RT> static matrix<T> rotation_x(RT);

    // ...

};

// ...

template <class T>
inline
matrix<T>
matrix<T>::translation(const vector<T>& v)
{
    matrix<T> result;
    result(0, 3) = v.x;
    result(1, 3) = v.y;
    result(2, 3) = v.z;
    return result;
}

template <class T, class RT>
inline
matrix<T>
matrix<T>::rotation_x(RT angle)
{
    RT ca = std::cos(angle);
    RT sa = std::sin(angle);
    matrix<T> result;
    result(1, 1) = +ca;
    result(1, 2) = -sa;
    result(2, 1) = +sa;
    result(2, 2) = +ca;
    return result;
}

As far as I can tell, those two implementations don’t differ much technically. The main difference is that rotation uses an extra template argument compared to translation

Yet, g++ doesn’t accept rotation the way I currently have it:

la/matrix.hpp:272:31: error: invalid use of incomplete type 'class la::matrix<T>'
la/matrix.hpp:16:7: error: declaration of 'class la::matrix<T>'

What’s going on here?

Answer 1

Your class only had one template parameter, and the second one refers to a template function of a template class, so you need

template <class T>
template <class RT>
inline
matrix<T> matrix<T>::rotation_x(RT angle) { .... }

This applies whether the functions are static or not.